This invention pertains generally to semiactive missile guidance systems, and in particular to an improved system for detecting and tracking an airborne narrow band noise jammer, thereby enabling the deleterious effects of such jammer to be countered.
As is known, narrow band jamming is a self-screening electronic countermeasure (ECM) technique (for use particularly against continuous wave (CW) or Doppler radars) in which a narrow band noise jammer produces a jamming signal centered about the carrier frequency of a victim radar. The narrow band noise will not be detected by the jammer analysis circuits of the victim radar because such circuits test for the presence of jamming signals from noise jammers by searching a frequency band removed from the band in which the carrier frequency falls. Consequently, the signals will not be generated to command the victim radar to enter the so-called "home-on-jam"(HOJ) mode, nor will the victim radar declare a coherent target track.
Even if the jammer analysis circuitry in a semiactive missile were effective to command the victim radar to enter the HOJ mode, a narrow band jammer of the type here contemplated would nevertheless continue to prevent proper guidance. Thus, when the HOJ mode is entered in a semiactive guidance system, a speed gate (Doppler tracking loop) is swept across a predetermined acquisition bandwidth (in the order of 60 kHz), in an attempt to acquire a tracking signal on jammer energy. If, however, bandwidth of the narrow band jammer is low compared to the predetermined acquisition bandwidth, then the apparent duty cycle would not allow tracking on the jamming signal.